I've heard a claim that ZFS relies too much on RAM caching, but
implements no sort of priorities (indeed, I've seen no knobs to
tune those) - so that if the storage box receives many different
types of IO requests with different "administrative weights" in
the view of admins, it can not really throttle some IOs to boost
others, when such IOs have to hit the pool's spindles.

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For example, I might want to have corporate webshop-related
databases and appservers to be the fastest storage citizens,
then some corporate CRM and email, then various lower priority
zones and VMs, and at the bottom of the list - backups.
AFAIK, now such requests would hit the ARC, then the disks if
needed - in no particular order. Well, can the order be made
"particular" with current ZFS architecture, i.e. by setting
some datasets to have a certain NICEness or another priority
mechanism?
Thanks for info/ideas,
//Jim
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